The home page does not validate.
I went to the ideal weight page and this does not validate either.
The home page has 111 errors and 2 warnings. The ideal weitht page has 49 errors and 3 warnings.
This site is very simple. Since it is right to the point I like the design. I would give it an 8 out of 10.
It asks you for a few peices of information about yourself and gives you a result based on those numbers entered. The website gives you a meter that shows you where you fall in the BMI range. This makes it easy to enter in numbers that you want to achieve and tells you when you are at an ideal weight or BMI. I would rate the usability a 9 out of 10.
The site's purpose is to educate the user on his/her health and it does do this very well based on the truthfullness of the numbers entered.